In the high-production cookware, appliance and automotive industries, as well as the low- and medium-production aircraft, aerospace, and job-shop industries, metallic sheet may be formed by a variety of different dies, the type and size of the die being dictated by the shape and intended use of the particular part. One process which is used to form a wide variety of these parts is the conventional drawing process. In a draw die, the blank is drawn across a binder surface allowing metal to flow from the bind surface and onto the part. Unfortunately, variable and non-uniform stresses are thereby developed throughout the part which results in localized stretching. This creates severe springback and shape retention problems which make it nearly impossible to predict, especially with large parts, the amount of springback that will occur. The common practice to overcome this springback or shape retention problem is to overbend (deform beyond the desired shape) the part. Finding the appropriate degree of overbend requires a number of costly trial and error procedures. There is also a significant amount of material waste in the drawing process because the blank is oversized to compensate for the metal flowing across the binder surface and into the die cavity.
In U.S. Pat. No. 4,576,030, a process is described wherein sheet metal can be one hundred percent stretch formed between co-acting male and female die halves. This is accomplished by providing a pair of opposed lock beads, at least one of which is provided with a number of spaced apart beads adapted to bite into the sheet metal, around the periphery thereof, when the gripper steels are closed. This permits the sheet metal to be homogeneously, one hundred percent stretch formed, thus resulting in a higher quality of shape retention, a reduction in the number of shock lines and stretch lines, less waste, and increased overall part strength.
Another procedure which enhances the quality of the formed part is fluid forming, that is, applying pressurized fluid against one side of the blank in the forming process. The benefits include increased versatility, a better finish on the final part, lower tool and reduced maintenance costs.
In U.S. patent application Ser. No. 07/855,815, entitled "Apparatus and Method for Hydroforming Sheet Metal," issued as U.S. Pat. No. 5,372,026 incorporated herein by reference, a process for stretch forming sheet metal by applying pressurized fluid against one side of the blank is described. The blank is 100% stretch formed into the part print cavity of the upper die. The process for stretch forming described involves placing the sheet metal in preferably, a conventional double action press. The gripper beads fitted to the upper and lower binders of the die are configured to bite into the sheet metal around the periphery to hold the blank in place and to seal it along the periphery. The type of gripper beads that were found to be particularly useful in gripping and sealing the sheet metal blank were those disclosed in U.S. Patent No. 4,576,030 described above. When the press is closed, the gripper beads are forced into the metal sealing its periphery. The liquid is then applied under pressure to the side of the sheet metal opposite from the die cavity configured for the part to be produced. The pressure of the liquid is sufficiently high to stretch form the sheet metal against the die cavity to produce the shaped part.
While these advancements have continued to improve the quality of the part and stretch the limits of product design, there are part configurations which cannot take advantage of 100% stretch forming. In particular, a part may have a configuration which, if the blank were 100% stretched, would cause thinning in areas where the elongation requirements of the configuration are above that of the blank material. In addition, tearing of the blank material may result.
It is desirable to provide specific tooling usable in a conventional double action press which combines the favorable aspects of fluid forming, the advantages of stretch forming and the flexibility of draw forming to permit a more accurate approximation of the desired part while reducing if not eliminating the problem of thinning or tearing of the blank material.
Another problem in using the process and apparatus of the prior art is that when large parts are being formed, enormous total hydraulic pressure is generated on the dies and transmitted to the press. For example, a car hood has generally about 2,000 square inches of area. If the desired forming pressure is 4,000 psi, then the resultant force on the dies is 2,000 square inches times 4,000 psi which equals 4,000 tons. Such force can deflect the die which spans across the outer blank holder opening sufficiently to cause the grippers to disengage. Even a slight deflection of the die can cause the gripper beads to disengage causing the hydraulic fluid to leak. To assure that the pressure of the liquid does not distort the shape of the die and cause leaks, high tonnage rated presses must be used. However, this significantly increases the cost of the operation. Additionally, conventional presses of sufficient tonnage may not be available for large parts that require high forming pressure.
It is desirable to provide a mechanism which locks the upper and lower dies securely together during the forming process. Such security allows lower tonnage presses to be used in the forming process.